1. Field of the Invention
This invention relates to a scanning optical system for scanning a laser beam on a scanning surface to form a pattern thereon, and particularly to an apparatus in which a pattern to be formed is high in accuracy.
2. Description of the Prior Art
In a conventional scanning optical system, as shown in FIG. 20, an intensity modulated laser beam is made incident to a polygon mirror 1 and the light beam reflected and deflected by the polygon mirror 1 is focused by an f.theta. lens 2 thereby to form a spot on a surface to be scanned. In general, the angle formed by an optical axis Ax1 of the light beam made incident to the polygon mirror 1 and an axis Ax2 of an f.theta. lens is approximately 50.degree..about.90.degree..
In order to obtain a scanned image- with high accuracy, the spot must be reduced. As the diameter of a spot becomes smaller, the focal depth (i.e., depth of field) becomes shallow. Therefore, in order to perform an image scan with high accuracy, the curvature of the field must be maintained as very small.
However, with the above-mentioned construction, the point of deflection on the polygon mirror, as shown in FIG. 21, is changed asymmetrically on both sides of the optical axis Ax2 of the f.theta. lens according to rotation of the polygon mirror. Therefore, there was a problem that the curvature of field becomes asymmetric on the surface to be scanned and correction can not be made when a lens symmetric with respect to the optical axis is used.
FIG. 22 shows a curvature of field for the construction of FIG. 20. The effective scanning field is 600 mm for an image height -300.about.+300 mm.
In order to reduce the curvature of field, it is effective to change the point of deflection symmetric with respect to the optical axis Ax2.
Therefore, as is shown in FIG. 23, a construction with a laser beam that is incident to a polygon mirror along the optical axis Ax2 of the f.theta. lens can be considered. However, in order to employ such a construction, it is required that a static deflector, for separating a light beam directed to the polygon mirror from a light source, from a beam light incoming to an f.theta. lens, is disposed in the optical path.
When a laser beam is made incident onto the polygon mirror along the optical axis Ax2 of the f.theta. lens, the curvature of field becomes as shown in FIG. 24.
As the appearance of the curvature of the field becomes symmetric, correction can be made using a symmetric lens with respect to the optical axis and the value of the spot become very small.
Japanese Patent Early Laid-open Publication No. Sho 60-233616 discloses a construction in which a light beam from a light source is made incident to a polygon mirror along the optical axis of a scanning lens using a polarizing beam splitter and a 1/4 wavelength plate.
However, according to the abovementioned Publication, the static deflector becomes complicated in structure and thus the cost increases. Furthermore, because the angle of incidence of the light beam to a polarized beam splitter is changed in accordance with rotation of the polygon mirror, the transmittance of the polarized beam splitter is gradually changed as it goes toward the peripheral portion from the central portion and irregularity in the light quantity occurs on the surface to be scanned.
FIG. 25 shows one example of the change of transmittance of a polarized beam splitter caused by the change in angle of incidence.
It is noted that in Japanese Patent Early Laid-open Publication No. Sho 60-233616, a laser beam is temporarily imaged on a reflecting surface of a polygon mirror in an auxiliary scanning plane in order to correct the adverse effect caused by inclination of the reflecting surface of the polygon mirror with respect to a rotational axis thereof.
Therefore, as the optical path in the auxiliary scanning plane becomes the same, both for the incident light beam to the polygon mirror, and the reflected light beam by the polygon mirror, the optical path can not be separated by a total reflection mirror, but a polarized beam splitter or a half mirror must be used.